Checking "!open" did not work with the NFS plugin because that plugin
does not close the file automatically, unlike CURL.

This reverts commit 966c4244.

The commit was bad, because the bug was really in
NfsInputStream::DoRead(); see previous commit.

Avoids off-by-one bug and obsoletes the bug fix in commit 966c4244

More robust and cancellable.

May cause assertion failure.

New base class for other InputStream implementations that run in the
I/O thread.

Fixes assertion failure.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Allow compiling test programs with only selected plugins.

Caused by commit 0b4fa41a

Make attributes const, don't allow partial initialization.
Instantiate the CdioParanoiaInputStream object at the end of
input_cdio_open().

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

For strrchr().  Fixes build failure.

These are the default values already.